The present invention relates generally to determining the remaining capacity of batteries or electrochemical cells of various chemistries and, more particularly, to determining the remaining capacity of batteries by measuring the voltage of different battery types and sizes under precise resistive pulse loads.
In the past, conventional battery testers were devices that would indicate the voltage of a battery subjected to a relatively small resistive load. The size or value of that resistive load might be varied depending on the type or size of the battery being tested. More complex testers may compare a battery""s voltage under load to fixed voltages along a resistance ladder, where those voltage values correspond to specific levels of remaining charge.
One deficiency of conventional battery testers is that accurate test results are often limited to a specific battery type or chemistry. This occurs because newer battery types or chemistries have significantly different discharge voltage profiles than earlier types. For example, the newer various lithium chemistries often have a relatively flat discharge profile wherein their open circuit voltage (OCV) remains nearly constant from 100% capacity to 20% remaining capacity. Alternately, the discharge profile for alkaline chemistries is a linear slope wherein their OCV decreases proportionately to remaining capacity. Generally, equally discharged batteries of identical voltage ratings, but different chemistries, will produce different results when compared to voltages along a fixed resistance ladder.
Another source of test result error with conventional battery testers is the amount of resistive load applied during the test. Many battery applications today are in electronic devices requiring relatively high power or having high momentary power demands. Examples include cameras with flash, digital cameras, and communication devices. These devices often have battery monitor circuits which will inhibit operation below a specific supply voltage. For accuracy, a battery tester must apply a load that is similar in magnitude and duration to that of typical applications for the battery.
Accordingly, it would be desirable for an electronic circuit to be developed which can accurately determine the remaining capacity of batteries of various chemistries and sizes by measuring their voltage under precise current loads. These current loads are similar to those experienced by the batteries under their normal operating conditions.
It would also be desirable for an electronic circuit to be developed that can be programmed with software algorithms which can be altered to suit a variety of battery testing applications, such as consumer, medical, military, communication or photographic. It is understood that such software could be written for batteries of different sizes and chemistries, as well as for primary and secondary cells.
It is also desirable that the electronic circuit automatically perform a precise pulse load test which will not harm the battery under test. The duration and repetition of this load test cycle may vary depending on the battery type being tested. By precisely timing the duration of the resistive load, overheating and damage to the battery under test is prevented and accurate, reproducible results are ensured from test to test.
Another desirable feature for the electronic circuit is that it can be easily used by untrained operators, whereby it easily computes and displays test results as a percentage of remaining battery capacity.
It is desirable for a battery tester circuit to have a power source independent from the battery under test. This enables accurate testing and precise voltage measurement of a wide range of batteries regardless of the state of charge of the battery under test. Nevertheless, it is desirable for the battery tester circuit to maintain testing accuracy regardless of the circuit""s power supply voltage.
In a first exemplary embodiment of the invention, a method of determining remaining capacity in a battery is disclosed as including the following steps: detecting the presence of a battery within one of a plurality of specified terminals; automatically initiating a timed pulse load test on the battery upon detection in a terminal; continuously passing current from the battery through a specified resistive load for the terminal; measuring a voltage of the battery while under the resistive load; comparing the measured voltage to a discharge voltage profile of the battery; and, computing the measured voltage as a percent of remaining battery capacity.
In a second exemplary embodiment of the invention, an apparatus for determining remaining capacity in a battery is disclosed as including: a microcontroller having stored therein a plurality of battery discharge profiles; a plurality of terminals connected to the microcontroller, wherein each of the terminals is identified as being applicable for a specified battery; a resistive load associated with each terminal; and, a power supply for providing a supply voltage to operate the microcontroller. A timed pulse load test is automatically initiated by the microcontroller upon detection of a battery on any of the terminals so that current from the battery is passed through the associated resistive load. A voltage for the battery is then measured, with the measured voltage being compared to a battery discharge profile for the battery so that a remaining battery capacity is computed therefrom.